An effective method in modulating thermally activated delayed fluorescence (TADF) emitters from green to blue emission: the role of the phenyl ring

Abstract

Developing efficient blue emitters with high performance and low cost is crucial for the further development of organic light-emitting diodes (OLEDs). Based on the two experimentally reported green thermally activated delayed fluorescence (TADF) emitters, which are thioxanthone derivatives consisting of carbazole as an electron donor and 9H-thioxanthen-9-one-S,S-dioxide (SOXO) as an electron acceptor with donor–acceptor (D–A) or donor–acceptor–donor (D–A–D) structures, two new blue TADF emitters are designed by simply inserting a phenyl ring between D and A units. The TADF processes of the four thioxanthone derivatives are studied systematically through first-principles calculations. The role of the introduced phenyl ring in the excited state properties of the designed molecules is explored by analyzing the changes in molecular geometries, frontier molecular orbital distributions, the lowest singlet–triplet energy splitting (ΔE_ST), the spin orbit coupling (SOC) constants, the radiative decay rates (k_r) and the nonradiative decay rates (k_nr), as well as the intersystem crossing rates (k_ISC) and reverse intersystem crossing rates (k_RISC). The results show that when incorporating phenyl units into the D–A and D–A–D structures, both high k_r and enhanced k_RISC are achieved in Cz-Ph-SOXO and DCz-DPh-SOXO, demonstrating that incorporating the phenyl unit in D–A and D–A–D structures is an efficient way for developing new SOXO-based TADF molecules. It is worth noting that the k_RISC values for Cz-Ph-SOXO and DCz-DPh-SOXO are significantly increased with respect to those of the experimental molecules. The present results would provide helpful guidelines for developing new SOXO-based TADF molecules experimentally.